Secondary batteries are highly applicable to a wide range of products and have electrical characteristics with high energy density. Such secondary batteries are applied not only to portable electronic devices but also to electric vehicles, hybrid vehicles, and electric power storage devices, driven by electric driving sources.
A battery pack applied to an electric vehicle and the like is configured so that a plurality of battery modules, each having a plurality of battery cells, are connected to obtain a high output. Each battery cell is an electrode assembly and may be repeatedly charged and discharged by an electrochemical reaction among components including a positive electrode current collector, a negative electrode current collector, a separator, an active material, an electrolyte and the like.
Meanwhile, along with an increased need for a large capacity structure and utilization as energy storage sources in recent years, there is a growing demand for a multi-module battery pack in which a plurality of battery modules, each having a plurality of secondary batteries connected in series and/or in parallel, are aggregated.
A battery pack of a multi-module structure is manufactured so that a plurality of secondary batteries are densely packed in a narrow space, and thus it is important to easily discharge the heat generated from each secondary battery. The secondary battery is charged or discharged by means of electrochemical reactions as described above. Thus, if the heat of the battery module generated during the charging and discharging process is not effectively removed, heat accumulation occurs. In addition, the deterioration of the battery module is promoted, and in occasions, ignition or explosion may occur.
Therefore, a high-output large-capacity battery module, or a battery pack having the same, requires a cooling device that cools battery cells included therein.
Generally, the cooling device is classified into two types of cooling device, namely an air cooling type and a water cooling type, but the air cooling type is more widely used than the water cooling type due to short circuit or waterproofing of the secondary battery.
Since one battery cell may not produce a large power, a commercially available battery module generally includes a plurality of battery cells as many as necessary so as to be stacked and packaged in a module case. In addition, in order to keep the temperature of the secondary battery at a proper level by cooling the heat generated while individual battery cells are producing electricity, a plurality of cooling fins corresponding to the area of the battery cells are inserted as a heat dissipating member throughout the battery cells. The cooling fins are made of aluminum. The cooling fins absorbing heat from each battery cell are connected to a single cooling plate to transfer the heat to the cooling plate. The cooling plate transfers the heat, received from the cooling fins, to a heat sink, and the heat sink is cooled by cooling water or cooling air.
However, if a cooling fin made of aluminum, it is difficult to make close contact with the battery cell. In addition, at the present, the battery module tends to have a gradually increasing current density. Also, if the battery module has a high energy density, the amount of heat generated from every battery cell becomes large, and so there is a limit in cooling the battery cells just with the cooling fins having constant thermal conductivity.